Battery-powered devices such as a cellular telephone offer freedom and mobility by allowing users the ability to operate devices without being connected to a power grid. As these devices integrate more features and push for higher performance, they consume more power and require longer battery life. The amount of charge required to recharge the battery thus increases but the longer the recharge time, the less useful a mobile device becomes. Thus, reducing the charging time of battery-powered devices increases the use and benefit of such devices.
It is conventional for a mobile device such as a cellular telephone to be charged through a data interface cable such as a Universal Serial Bus (USB) cable. A USB cable has a current limit that cannot be exceeded for safety reasons. A power adapter such as a flyback converter that drives the USB cable to recharge the mobile device's battery must thus not exceed the current limit, which undesirably lengthens the charging time. To decrease the charging time despite the current limits of USB cables, certain fast charging protocols have been developed in which the output voltage driven from the power converter through the USB cable is elevated from a default level (for example, from 5V to 9V or 12V etc.) to deliver higher power without increasing the current above the charging cable limits. But such increased voltages require the mobile device to include a DC-DC converter to reduce the received voltage to a suitable level for battery charging. The DC-DC converter generates heat that limits the charging time reduction. In addition, the mobile device manufacturing cost is increased by the inclusion of the DC-DC converter.
To eliminate the need for a DC-DC converter in the mobile device, direct charging techniques have been developed in which the flyback converter directly charges the battery of the mobile device. The mobile device thus either does not include such an intervening DC-DC switching power converter or bypasses its use during a direct charging mode. In addition, modem USB cable current limits have been increased to, e.g., 4 Amps such that direct charging offers a desirably decreased charging time. A direct charging of a discharged battery typically proceeds over three phases. In a first phase, the battery voltage is low such that the direct charging occurs in a constant-current mode that drives the battery with the maximum allowable current over the USB cable (e.g., 4 A). The discharged battery's voltage gradually rises over the first phase until it reaches a battery voltage limit such as 4.3 V. The direct battery charging then enters a second phase involving a series of constant-current modes in which the current limit is successively decreased in steps from the USB current limit to a minimum reduced minimum current such as 2 A. With each reduction in the current limit, the battery voltage first drops and then increases until it reaches the voltage maximum, whereupon a lower current limit is enforced. When the battery voltage again reaches the voltage maximum with the current limit at the minimum limit for the second phase, the direct battery charging procedure enters into a third phase of constant voltage operation at the voltage maximum. The current slowly decreases during the third phase until the battery is fully charged.
But modern smartphones tend to be quite expensive such that it would be disastrous if the power adapter damages the smartphone's battery during the direct charging process. In that regard, the output voltage and output current from the power adapter must typically be closely monitored during the direct charging process. The output voltage may be monitored by the flyback converter's secondary-side controller using an analog-to-digital converter (ADC). But the monitoring of the output current typically requires the use of a sense resistor in series with the transformer's secondary winding so that the output current may be sensed by an ADC and reported to the mobile device. Given the high output currents (e.g. 4 A) used in direct charging, even a very small resistance (e.g., in the tens of mill-ohms) for the sense resistor introduces a significant power loss.
Thus there is a need in the art for flyback converters configured for direct charging without the use of a secondary-side sense resistor.